1. Field of the Invention
This invention relates to a spring steel which undergoes very little decarburization during hot working or heat treatment.
2. Description of the Prior Art
In general, flat springs, coil springs or torsion bars are manufactured by quenching and tempering hotrolled steel. If decarburization occurs during these manufacturing processes, however, the fatigue strength of the steel considerably deteriorates and it no longer has the properties required for a spring material. Conventionally, therefore, as described in Japanese Patent Application Laid-Open No. 54-136518, the steel has been subjected to heat treatment after forming a coating of an anti-decarburizing agent on the steel surface, or alternatively to controlled atmosphere heat treatment, etc., in order to prevent this decarburization from occurring.
Other methods of reducing decarburization consist in adding various elements to the steel. For example, these elements are V, Nb, Mo, W, Ta, Zr and Ti (Japanese Patent Application Laid-Open No. 59-177352); Cu and Ni (Japanese Patent Application Laid-Open No. 61-170542); As, Sn and Sb (Japanese Patent Application Laid-Open No. 61-183442); and Sb (Japanese Patent Application Laid-Open No. 1-319650).
The following conventional anti-decarburizing methods involve the following problems which inevitably lead to increased cost:
a: Forming a coating of an anti-decarburizing agent on the steel surface takes time, and requires special equipment. PA1 b: Controlled atmosphere heat treatment requires extra cost, and the cost of running the heating furnace is also higher than in the conventional case.